Imprint Resin Research Articles

Fabrication of Large-Area Nanostructures Using Cross-Nanoimprint Strategy.

Nanostructures with sufficiently large areas are necessary for the development of practical devices. Current efforts to fabricate large-area nanostructures using step-and-repeat nanoimprint lithography, however, result in either wide seams or low efficiency due to ultraviolet light leakage and the overflow of imprint resin. In this study, we propose an efficient method for large-area nanostructure fabrication using step-and-repeat nanoimprint lithography with a composite mold. The composite mold consists of a quartz support layer, a soft polydimethylsiloxane buffer layer, and multiple intermediate polymer stamps arranged in a cross pattern. The distance between the adjacent stamp pattern areas is equal to the width of the pattern area. This design combines the high imprinting precision of hard molds with the uniform large-area imprinting offered by soft molds. In this experiment, we utilized a composite mold consisting of three sub-molds combined with a cross-nanoimprint strategy to create large-area nanostructures measuring 5 mm × 30 mm on a silicon substrate, with the minimum linewidth of the structure being 100 nm. Compared with traditional step-and-flash nanoimprint lithography, the present method enhances manufacturing efficiency and generates large-area patterns with seam errors only at the micron level. This research could help advance micro-nano optics, flexible electronics, optical communication, and biomedicine studies.

Selective dry etching of UV-nanoimprinted resin passivation masks for area selective atomic layer deposition of aluminum oxide

To demonstrate area selective atomic layer deposition (ALD) using UV-nanoimprinted resin patterns as physical passivation masks, we investigated the removal of UV-cured resin films subjected to sequential mutual doses of trimethylaluminum (TMA) and H2O by using dry etching procedures. On the basis of the removal of a residual layer characteristic from imprint resin patterns by anisotropic oxygen reactive ion etching (O2 RIE), oxidatively etched UV-cured films were modified with TMA and H2O in an ALD-like cyclic manner. Atomic force microscopy and time-of-flight secondary ion mass spectrometry [time-of-flight secondary ion mass spectrometry (TOF-SIMS)] analysis suggested that the combination of physical Ar ion milling and subsequent chemical O2 RIE enabled the elimination of oxidized UV-cured resin masks modified with 5-cycle TMA doses. By contrast, Ar ion milling or O2 RIE left organic or inorganic residues on silicon surfaces, respectively. A TMA-modified hybridized resin layer was etched by physical Ar ion milling; subsequently, the organic residual resin layer was removed by chemical O2 RIE in the case of 5-cycle modification with TMA. The mapping image of Al+ visualized by TOF-SIMS suggested that line patterns of aluminum oxide were left selectively on unmasked silicon substrate surfaces by site-selective dry etching of TMA-modified imprint resin passivation masks with 500 nm linewidth.

Suppression of resist pattern collapse by crosslinker in ultraviolet nanoimprinting involving sequential infiltration synthesis with trimethylaluminum

The fabrication of resist patterns using UV nanoimprinting is required on consideration of the reduction of the use of hydrocarbons along recent amendments for environmental sustainability. In this study, we investigated the generation of resist pattern defects through UV nanoimprinting in a readily condensable trans-1,3,3,3-tetrafluoropropene (TFP) gas with a low global warming potential for elimination of nonfill defects arising from a bubble trap and subsequent sequential infiltration synthesis (SIS) to result in a vapor phase organic-inorganic hybridization for dry etching durability. A bisphenol A-based UV-curable imprint resin enabled the nanostructure fabrication of resist patterns without any nonfill defects in TFP; however, SIS consisting of subsequent mutual doses of trimethylaluminum and water caused a resist pattern collapse of 100-nm-height patterns with linewidths of <60 nm. A crosslinker with six acrylate moieties was selected based on its low TFP absorption. The crosslinker-containing imprint resin decreased the resist pattern collapse during SIS. Nanoindentation measurements suggest that the resist patterns made using the crosslinker-containing imprint resin were strengthened at 100 °C to carry out an SIS.

Single pulse development of chromium-deposited imprint micro/nano patterns of photo-cured crosslinked resin using a femtosecond pulsed laser

Exposure to a single pulse generated from a femtosecond pulsed laser for a duration of 100 fs at a wavelength of 800 nm and a laser fluence of 0.31 J cm−2 resulted in chromium (Cr) deposition on photocured insoluble imprint resin patterns to be ablated selectively; however, Cr deposition on synthetic quartz surfaces was retained. Single-pulse development of Cr-deposited imprint micro/nano patterns occurred at the same laser fluence, independent of the presence of a laser absorbing dye. Laser ablation of the cured resin near the Cr deposition was confirmed by atomic force microscopy. Depth profile analyses confirmed that Cr species were partially embedded in the cured resin underneath the layer of Cr. These results suggest that the mechanism of laser ablation was due to the rapid thermal expansion of resin near the embedded Cr species and deposited Cr layer rather than the multi-photon absorption of the cured resin.

The GaN-Based Light Emitting Diode Grown on Nanopattern Sapphire Substrate Prepared by Inductively Coupled Plasma Etching

We applied nanoimprint lithography to fabricate nanopattern sapphire substrate. Because the imprint resin cannot endure the inductively coupled plasma bombardment, we use Ni metal to replace the resin as our etching mask. And then, we tuned the parameters to achieve a depth-enough nanopattern sapphire substrate (200-500-nm thick). Then, light emitting diode (LED) structures were grown and the quality was characterized by X-ray diffraction and photoluminescence. The LED tester and integrating sphere were used to analyze the device property. The results could demonstrate that the film quality was improved with increasing etching depth. And, the compressive strain was also released with increasing etching depth. In addition, increasing etching depth could not only improve internal quantum efficiency, but also improve light extraction efficiency. Thus, the external quantum efficiency was enhanced. These evidences demonstrated that nanopatterned sapphire with deeper depth certainly enhanced performance of LEDs.

Sub 50 nm Nano-Patterns with Carbon Based Spin-On Organic Hardmask

Carbon based spin-on organic hardmask (C-SOH) was used as an imprint resin to fabricate sub 50 nm sized patterns. Imprinting of C-SOH was done with a polyurethaneacrylate (PUA) stamp. Patternability and etch resistance of the C-SOH resin was compared to poly(methyl methacrylate) (PMMA). C-SOH can be patterned at the nanosize using imprint lithography and exhibits superior etch resistance, especially for F-based plasmas. Due to the poor etch resistance of imprint resin such as PMMA, it is seldom used as an etch mask to form nano-structures by etching the Si3N4 layer. However, such a nano-structure was able to be formed by etching the Si3N4 layer using C-SOH as an etch mask.

Fabrication of ZnO nano-structures using UV nanoimprint lithography of a ZnO nano-particle dispersion resin

Recently, nanoimprint lithography (NIL) has gained great attention as an effective patterning technology in the fields of light emitting diodes (LEDs), solar cells, and other optical devices, because of its simplicity and cost effectiveness. The aim of this study is the development of an imprint resin containing dispersed zinc oxide (ZnO) nano-particles that is applicable in the UV NIL process. UV NIL uses conventional monomer-based resins, which contain a UV initiator, but restricts the use of imprinted structures in optical devices due to their relatively low refractive index. In order to resolve this problem, an imprint resin containing dispersed ZnO nano-particles was prepared, using which submicron-scale structures were fabricated by the UV NIL process. The haziness of submicron-scale ZnO nano-particle resin structures and the refractive index of the ZnO nano-particle dispersion resin were measured to analyze the optical properties of the ZnO nano-particle dispersion resin and the resulting structures.

Improvement of photon extraction efficiency of GaN-based LED using micro and nano complex polymer structures

A micro- and nanoscale complex structure made of a high refractive index polymer (n = 2.08) was formed on the ITO electrode layer of an edge-emitting type GaN blue light-emitting diode (LED), in order to improve the photon extraction efficiency by suppressing total internal reflection of photons. The nanoimprint lithography process was used to form the micro- and nanoscale complex structures, using a polymer resin with dispersed TiO2 nano-particles as an imprint resin. Plasma processing, such as reactive ion etching, was used to form the micro- and nano-scale complex structure; thus, plasma-induced damage to the LED device can be avoided. Due to the high refractive index polymeric micro- and nanostructure on the ITO layer, the electroluminescence emission was increased up to 20%, compared to an identical LED that was grown on a patterned sapphire substrate to improve photon extraction efficiency.

Functional SiO2–TiO2–PEG hybrid resin by direct chelation with catecholic compound

The mesoporous inorganic–organic SiO2–TiO2–PEG hybrid resin systems with various useful functionalities were directly synthesized by a binary sol–gel reaction of TEOS–TiCl4 and the subsequent chelation with a chatecholic compound (dihydroxy benzene), dihydroxy-m-benzenedisulfonic acid disodium salt, on the surface Ti ion of the ordered mesoporous SBA-15 network structure, respectively. Moreover, the hybrid resins consisting of polyethylene glycol and a silane coupling agent exhibited the controlled wettability, excellent coating processibility on various substrates with strong abrasion resistance. Furthermore, the transparent and low viscous resin showed the successful performance to fabricate various nanoscale patterns with the feature size down to 170 nm by imprint lithography. Based on the excellent patternability, nanofluidics with 100 nm of the narrowest dimension channel height was fabricated to employ a capillary electrophoresis for separation DNAs without gel matrix. In addition, the presence of sulfonic acid in the resin also showed the solid acid catalytic performance. These results reveal that the developed hybrid materials are very useful as an imprint resin as well as versatile microchemical applications.

Fabrication of Sub-50 nm Au Nanowires using Thermally Curing Nanoimprint Lithography

As narrow as 50 nm metal nanowire array patterns were successfully fabricated by nanoimprint lithography (NIL) using thermally curable monomer based resin. Compared to conventional hot embossing lithography, which has used thermoplastic polymers such as PMMA as an imprint resin and thus process temperature and pressure are over 180°C and 50 bar, respectively, thermally curable monomer based resin enabled imprint process at relatively lower temperature (120°C) and pressure (20 bar). Due to the highly fluidic nature of monomer based liquid phase resin, residual layer was not observed after imprinting. Imprinted resist pattern was then treated with oxygen plasma for 5 seconds and 5 nm of Ti layer and 15 nm of Au layer was deposited by e-beam evaporation. The imprinted resist pattern was lifted-off by dipping into an organic solvent, such as acetone. As the result, as narrow as 50 nm Au nanowire array pattern with area of 30 mm × 40 mm was fabricated on a Si substrate. 30 mm × 40 mm of nanowire pattern area was not limited by nanoimprint process, but the laser interference lithography process, which was used to fabricated the master template for imprinting.

Direct indium tin oxide patterning using thermal nanoimprint lithography for highly efficient optoelectronic devices

Indium tin oxide (ITO) is the most commonly used transparent conducting oxide (TCO) material on account of its high conductivity and transmittance in a visible range. In order to improve the efficiency of optoelectronic devices using TCO materials, the transmittance and conductivity of the TCO layers need to be improved. Recently, various techniques, which use nanostructures on the surface to improve the transmittance, have attracted considerable attention. In this study, a direct ITO nanopatterning technique using thermal nanoimprint lithography was presented. An ITO nanoparticle solution is used as an imprint resin and postimprint annealing process was followed. The optical transmittance of the ITO glass was increased by forming periodic ITO dot patterns on the ITO glass. UV-visible near infrared spectra showed that the transmittance at 485nm of the ITO glass with the directly patterned ITO layer was 5% higher than that of ordinary ITO glass.

Fabrication of polycrystalline TiO 2 nanopatterns by TiO 2 sol base imprint lithography

This paper reports a process for fabricating TiO 2 nano-patterns using nanoimprint lithography and a sol–gel method. An ethanol-based TiO 2 sol was prepared using tetrabutylorthotitanate as a precursor and used as an imprint resin. A replicated polydimethylsiloxane (PDMS) mold was used as an imprint stamp. During the imprinting process at 5 atm and 200 °C for 1 h, the TiO 2 sol changed to a TiO 2 gel by absorbing the solvent into the PDMS mold. After imprinting, a TiO 2 gel pattern was formed on an oxidized Si wafer. After subsequent annealing, it confirmed that patterns of the master template were transferred to TiO 2 patterns by Scanning Electron Microscopy. Furthermore, Transmission Electron Microscopy and X-Ray Diffraction showed that the TiO 2 gel patterns had been converted to an inorganic polycrystalline TiO 2 pattern.

Lift-off process using bilayer ultraviolet nanoimprint lithography and methacryloxypropyl-terminated-polydimethylsiloxane-based imprint resin

A high-fidelity lift-off process using bilayer ultraviolet nanoimprint lithography (UV-NIL) to fabricate nanosized metal pattern was demonstrated. High O2 reactive ion etching (RIE) selectivity of the imprinted resist pattern to the underlayer is necessary to create the undercut pattern profile that is essential for the stable lift-off process. A methacryloxypropyl-terminated polydimethylsiloxane (M-PDMS)-based UV-curable liquid-phase imprint resin and a polyvinyl alcohol (PVA) underlayer were applied to bilayer UV-NIL for the high-fidelity lift-off process. The M-PDMS-based imprinted resist pattern showed high etch resistance to O2 plasma and the undercut pattern profile of the PVA underlayer for the lift-off process was formed by the O2 RIE process. The size and shape of the imprinted resist patterns were hardly changed during PVA underlayer etching by O2 RIE. As a result, high-fidelity and high-aspect-ratio metal patterns without rabbit-ear-shaped defects were fabricated by the lift-off process using bilayer UV-NIL with the M-PDMS-based imprint resin, which has high etch resistance to O2 RIE, and the PVA underlayer.

Mimicking the Nanostructure of Bamboo Leaves (Backside) for Hydrophobicity Using Polydimethylsiloxane Moulding and Nano-Imprint Lithography

Extensive studies have revealed that various kinds of plant leaf have a hydrophobic property which arises from the micro- and nano-scale structure of the leaves. As the self-cleaning capability of plant leaves, termed the lotus effect, is based on their micro- and nano-scale structure, this hydrophobic property can be obtained on various other surfaces by duplication of the leaves' structure. In this study, the hydrophobic structure on bamboo leaves (Pseudosasa japonica) was successfully replicated on a glass substrate using polydimethylsiloxane (PDMS) molding technique and UV nano-imprint lithography. The replicated nano structure, made of perfluorinated acrylate imprint resin, was characterized by scanning electron microscopy (SEM) and its hydrophobicity was evaluated by contact angle measurements which confirmed that the hydrophobic nature and self-cleaning capability of the original bamboo leaves were also replicated.

The direct nano-patterning of ZnO using nanoimprint lithography with ZnO-sol and thermal annealing

Nano-patterned ZnO layer was fabricated by ZnO-sol imprinting with a polymeric mold, followed by annealing. Instead of polymer based imprint resin, ZnO-sol was used as an imprint resin. During the imprinting process, the organic solvent in the ZnO-sol was absorbed into a polymeric mold and thus, ZnO-sol was converted to ZnO-gel. These patterns were subsequently annealed at 650 °C for 1 h in atmospheric ambient to form ZnO patterns. X-ray diffraction (XRD) and photoluminescence (PL) confirmed that ZnO-gel was completely converted into ZnO by annealing. Using this ZnO-sol imprinting method, ZnO nano-patterns, as small as 50 nm, were fabricated on Si and oxidized Si wafer substrates. The ZnO nano-patterns were characterized using scanning electron microscopy (SEM) and Transmission electron microscopy (TEM).

Direct imprint of conductive silver patterns using nanosilver particles and UV curable resin

The direct nanoimprinting of device patterns on functional resist has attracted increasing attention since it has the potential to extend nanoimprinting applications and improve productivity. Considerations of device-oriented functional resins are currently limited to optical or optoelectronic applications where the optical properties of the imprint resin can be selectively controlled. Either nanosilver particle dispersions or conductive polymers are candidate resins for the direct imprinting of electrode line patterns. In this paper, we describe direct ultraviolet (UV) imprinting using acrylate-based resin incorporating nanosilver colloid, with optimized post-imprinting steps involving heat treatment and wet etching. Resin mixtures with 40, 50, and 60 wt% of nanosilver loading were prepared, and UV imprinting was performed on the dispensed resin mixtures, followed by heat treatment to sinter the nanosilver particles and wet etching of the imprint residues. The pattern shrinkage that occurred during the post-imprinting steps reached 25–35%, and was strongly dependant on the nanosilver concentration. The electrical resistivity for 60 wt% Ag loading was roughly 2.5 times higher than that of bulk silver. In addition, surface texturing effects on the imprinted patterns were investigated using selective oxygen plasma etching.

Fabrication of 100nm Sized Patterns on Flexible Polyethylene-Terephthalate Substrate Using Monomer Based Thermal Curing Nanoimprint Lithography

In order to build a nano-device on polymer substrate, nano-size patterning must be done. However, conventional photolithography cannot be used to fabricate nano-sized patterns on polymer film due to the flexibility of polymer film and its potential interaction with developer solution and organic solvent. In this study, 100nm sized dense line and space patterns were made on flexible PET (polyethylene-terephthalate) substrate using newly developed monomer based imprinting lithography. Compared to hot embossing lithography, thermal curing imprint lithography uses monomer based imprint resin which consists of base monomer and thermal initiator. Since it is liquid phase and polymerization temperature is much lower than glass temperature of polymer, the nano-sized patterns can be transfer at much lower temperature and pressure. Hence, patterns as small as 100nm were successfully fabricated on flexible PET film substrate by monomer based thermal curing imprinting lithography at 85°C and 5atm.

Nano Sized Patterning on the Thermoset Materials Using Thermal Curing Nano-Imprinting Technology

Nano-imprinting technology, currently developing as an alternative technology for photolithography, is mostly dealing with photo resist materials for etching barrier. In other words, the materials imprinted are used as the barriers for wet etching or RIE (reactive ion etching), and then are removed after all. Thus, most materials developed for nano-imprinting technology have relatively low Tg(<100oC), in case of solids or have a low viscosity before curing in order to fill the gap of the stamp. Such materials could not be suitable for the structures that require relatively high temperature process ability, high modulus or high durability. Epoxy is one of candidates that meet such requirements for the structures. However, it has not been reported about imprinting on epoxy materials. In this paper, nano-imprinting technology was developed to fabricate the microstructures. Instead of thermoplastic imprint resin, solid phase epoxy films, which is thermoset polymer, were imprinted with SAM (self assembly monolayer) coated quartz and silicon stamps. Feature size of stamp varies from 150nm to 1 μm and height was 300nm for quartz stamp and 3μm for Si stamp.

Fabrication of nano-sized resist patterns on flexible plastic film using thermal curing nano-imprint lithography

Due to polymer’s excellent flexibility, transparency, reliability and light weight, it is a good candidate material for substrate of devices including organic electronic devices, biomedical devices, and flexible displays (LCD and OLED). In order to build such devices on polymer, nano- to micron-sized patterning must be accomplished. Since polymer materials reacts with organic solvents or developer solutions which are inevitably used in photolithography and cannot bear high temperature (∼140 °C) process for photoresist baking, conventional photolithography cannot be used to polymer substrate. In this research, monomer based thermal curing imprinting lithography was used to make as small as 100 nm dense line and space patterns on flexible PET (polyethylene-terephthalate) film. Compared to hot embossing lithography, monomer based thermal curing imprint lithography uses monomer based imprint resin which consists of base monomer and thermal initiator. Since it is liquid phase at room temperature and polymerization can be initiated at 85 °C, which is much lower than glass temperature of polymer resin, the pattern transfer can be done at much lower temperature and pressure. Hence, patterns as small as 100 nm were successfully fabricated on flexible PET film substrate by monomer based thermal curing imprinting lithography at 85 °C and 5 atm without any noticeable degradation of PET substrate.

Effect of imprinting pressure on residual layer thickness in ultraviolet nanoimprint lithography

Since ultraviolet (UV) imprint lithography can be used to fabricate nanosized patterns with extremely high throughput, it is regarded as one of the most promising technologies for nanosized patterning. However, for it to become a mass production technology for the fabrication of nanosized patterns, UV imprint lithography must demonstrate the capability to perform imprinting with a uniform and minimized residual layer. In UV imprint lithography, the thickness of the imprint residual layer is determined by various process parameters, including the initial thickness of the applied resin layer dependent on the spin speed, the viscosity of the resin, the imprint pressure, and the structure of the imprint template and pattern area. Among these process parameters, the thickness of the applied resin layer and the imprint pressure are focused on in this study. In order to transfer patterns with a uniform and minimized residual layer, the imprint resin must be rearranged during imprinting. Thus, the ability to achieve uniform pressing with a high pressure (>15atm) is very important. In this study, a commercially available imprinting system was used. The imprint system is of the pressurized chamber type and uses isotropic pressure to deliver the pressing force uniformly. A commercially available UV curable imprint resin was used.